A recent thread was discussion reproduction lower trim and today I happened through the Osborn Assembly manual and ran across the seat assembly drawings. It may help narrow down the change-over dates some.
The 67 Osborn Assembly manual drawing shows the updated version with the side screw & "T" bracket dated as 10-10-66 which supersedes 7-11-66 so this would indicate the plan went into effect Mid-October for the later design. My very early November car came with the early version (no side screw or "T" bracket) so I would say the change-over most likely occurred as stock depleted and as new inventory arrived (November built cars?).
I would be curious what other NOVEMBER built examples have.